(ROSEMOUNT, Minn.) -- A mother and daughter died when a roughly 800-pound boulder fell off a truck and smashed into their car, and now that truck driver is in custody, according to police.

Karen Christiansen, 67, was driving with her daughter, Jena Christiansen, 32, in Rosemount, Minnesota, Monday afternoon when the massive boulder dislodged from the back of the truck and crashed into their 2002 Toyota Avalon, the Rosemount Police Department said.

The mother and daughter, both of Shoreview, Minnesota, died from blunt force head trauma, the Hennepin County Medical Examiner's office said.